Electronic components are commonly soldered to printed circuit (PC) boards with a lead-tin solder. A maximum soldering temperature of 260° C. (500° F.) has become a standard in the industry and this limit has propagated to many other parameters. For example, most components to be soldered to printed circuit boards are rated for a maximum temperature of 260° C. Continuous soldering apparatus is built to operate at a maximum temperature of about 260° C. Even the printed circuit (PC) boards (sometimes called printed wiring boards, PWB) are generally constructed for a maximum soldering temperature of about 260° C.
There is a desire to eliminate hazardous lead from solder, and there are even moves afoot to ban the use of lead. Lead-free solder will be required in many products which now use lead-tin solder. Exemplary substitute lead-free solder alloys include tin-silver and tin-silver-copper alloys having about 95-96.5% tin and 3.5-5% silver. (There is a eutectic at 3.5% silver in the binary Ag—Sn phase diagram.) Copper is often in the range of about 0.3 to 1%. Some tin based solders have been proposed with additions of antimony, bismuth, indium, nickel and/or zinc. Tin is the base for the lead-free solder alloys and is typically present as more than 90% of the alloy.
Soldering processes have been developed which make automatic soldering of PC boards highly reliable. Plated-through holes are filled, ample solder fillets are almost always found, and bridging between closely spaced connector points is rare. To achieve similar reliability with lead-free solders such as tin-silver solder alloys, it is generally found that soldering temperatures of 270 to 275° C. (520° F. or higher) are necessary. Clearly this is higher than the conventional 260° C. limit and has the potential for damaging components. Therefore, reducing the temperature for soldering with such lead-free substitute alloys is highly desirable, particularly in view of the coming requirement for use of lead-free solder.
Another issue which is of concern with respect to both the lead-tin solders and substitute solder alloys is accumulations of dross on the solder. Dross is an accumulation of oxides of the metals in the solder. It can form a solid crust on the molten solder as it accumulates during operation of soldering apparatus. Sometimes it is appropriate to shut down continuous operating apparatus and manually ladle dross from the solder bath. Even when not shut down, manual removal of dross from the surface of the hot solder is practiced. Substantial amounts of solder can be lost into the dross, which then needs to be processed to recover and recycle the metal. Even when dross is not visible, a small amount on the surface of the molten solder can lead to bridging of solder between closely spaced leads and/or failure to wet surfaces to be soldered, so that incomplete or poor joints are obtained.
Due to study of this invention, we are now confident that purity of the solder bath is an important factor in difficulty with soldering. It appears that metal oxide distributed in the bath interferes with wetting and successful soldering. The oxide may raise solder viscosity, provide nucleation sites for crystallization at higher temperatures than solidification in absence of such oxides, and may cause weakness in solder joints. Thus, in addition to visible dross on the surface, a significant issue is purity of the molten solder bath.
It is found in practice of this invention that formation of dross in continuous soldering apparatus can be significantly minimized or even eliminated by durable additives. Most surprising, the temperature at which viable soldering takes place with lead-free solder alloys has been reduced by as much as 30° F. (16 to 17° C.). Soldering temperature for tin-silver alloys can be brought below the 260° C. limit.
Furthermore, there is a surprising reduction in viscosity of the molten metal in a wave solder apparatus, for example. This may contribute to the excellent solder joints obtained at plated-through holes in PC boards. Such improvements in solder joints are also due to better wetting as shown by wetting balance tests. Cleanliness of the solder bath is believed responsible.
A variety of wave soldering, fountain soldering and cascade soldering systems which may be used in practice of this invention are described and illustrated in ASM Handbook, Volume 6, Welding, Brazing, and Soldering. Exemplary apparatus, as illustrated in FIG. 7 which is largely copied from Metals Handbook, page 1088, comprises a large vat or “solder pot” in which molten solder 10 may be held at the desired soldering temperature. A pump (not shown) draws solder from near the bottom of this molten mass and forces it upwardly through one or more slot nozzles 11 from which the solder flows laterally like a waterfall, either in one direction or both directions from the slot, and back into the vat. The upper surface of the flowing solder is commonly referred to as a “wave”.
When such a wave soldering apparatus is used for soldering, a printed circuit board 12 is moved across the apparatus so that the lower face of the PC board contacts the upper surface of the wave 13 of molten solder. Molten solder wets the surfaces to be soldered, and wicks into the plated-through holes and around leads, and makes good solder joints therebetween. In such automatic apparatus PC boards are fed into the wave in close succession for high capacity production. There are also so-called fountain soldering machines and cascade soldering systems with which this invention is useful.
Sometimes a portion of the solder in wave soldering apparatus overflows into a secondary reservoir and molten solder returns from the reservoir to the larger solder pot. Dross forming on the solder due to oxidation upon exposure to air also overflows and accumulates in the secondary reservoir, from which it may be removed. Some dross also may flow along the surface of the wave. There is appreciable turbulence where such “waterfalls” of molten solder meet the surface of the solder bath, providing surfaces where metal oxides or dross may form. The deleterious effects of such phenomena are ameliorated by this invention.
In practice of this invention, a sufficiently extensive liquid active additive layer is maintained on the molten solder bath during the soldering process for maintaining purity or cleanliness of the bath. The layer provides the surprising result of significantly lowering the temperature at which reliable solder joints are obtained. The liquid layer preferably comprises a material that is stable at the temperature of the bath, effectively bars oxygen in air from reaching a quiescent surface of the bath, and has the ability to assimilate oxide of at least one metal in the bath and remain liquid for a commercially acceptable time. Typically, the material comprises an organic molecule with nucleophilic and/or electrophilic end groups. Carboxylic —COOH end groups are particularly preferred.
An exemplary substance comprises a dimer acid such as described in greater detail hereinafter. A dimer acid has previously been used as a cover or oxygen barrier material on the surface of a bath of molten metal as lead and tin are melted together to formulate a lead-tin solder alloy. Minor amounts of dimer acid have been formulated into soldering flux compositions.